1. Field of the Invention
The present invention relates to the storage of hydrogen on an industrial scale. It relates more specifically to materials which permit a reversible storage of hydrogen in the form of metal hydrides.
2. Description of the Related Art
Hydrogen (H2) is used in numerous industrial fields, especially as a fuel (for example, in heat engines or fuel cells), or also as a reagent (typically for hydrogenation reactions). In this context, bearing in mind its volume in the gaseous state and its explosiveness, it is desirable for hydrogen to be stored in a form ensuring a small space requirement and safe containment.
Hydrogen is usually stored under pressure (so-called hyperbaric storage at pressures typically of the order of from 20 to 70 MPa) or in liquid form (at temperatures lower than or equal to 20.4 K). These storage methods are generally found to be expensive, particularly in terms of energy. In addition, they do not enable safety requirements to be fully satisfied, especially in the case of storage under pressure.
More advantageously, it has been proposed to store hydrogen in the form of hydrides. In this context, the hydrogen to be stored is typically brought into contact with a metallic material (generally an alloy) under pressure and temperature conditions which cause the hydrogen to be incorporated in atomic form in the crystal lattice, by conversion of the molecular hydrogen H2 into a hydride (so-called hydrogen “charging” step). In order to recover the hydrogen thus stored, conditions of lower pressure and/or higher temperature, which promote the reverse reaction (hydrogen “discharge”), are required. In this context, it is possible to determine a “reversible storage capacity”, expressed as a percentage by mass, which corresponds to the maximum amount of hydrogen which can be discharged by the storage material once it has been charged. For more details on the storage of hydrogen in the form of hydrides, reference may be made especially to Hydrogen in Intermetallic Compounds I and II, L. Schlapbach, Springer-Verlag, (1988).
Compared with the above-mentioned storages under pressure or at very low temperature, the above-mentioned storage of hydrogen in the form of hydrides permits safer storage, with a smaller space requirement and generally a lower cost, especially in terms of energy. Furthermore, hydrogen freed from hydrides has the advantage of being in a particularly pure form, which makes it especially suitable for use in devices of the fuel cell type or in fine chemistry reactions where it is desired to be free from the presence of impurities to the maximum extent.
In particular, the use of materials having a crystal structure of type AB2 (such as ZrCr2), or also materials comprising alloys of the type FeTi or LaNi5, as metal compounds capable of ensuring the storage of hydrogen in the form of hydrides has been described (in this connection, reference may be made especially to the above-mentioned work Hydrogen in Intermetallic Compounds I and II). However, those materials are of limited interest because, although they lead to the above-mentioned advantages, they have mediocre performances, especially in terms of reversible storage capacity. In particular, athough the alloy FeTi is relatively inexpensive, it has a low reversible storage capacity (of the order of 1% by mass), which means that it is used only very selectively, for heavy-duty applications (for example in submarines). The LaNi5 alloys for their part have much higher manufacturing costs with a reversible storage capacity which is still low (generally of the order of 1.4% by mass at the very most). As for materials having a crystal structure of type AB2, their reversible storage performance is generally lower than 1.8% by mass and, in addition, they usually have stability problems after a few cycles of hydrogen charging and discharging.
More advantageously, in order to effect the reversible storage of hydrogen in the form of hydrides, it has been proposed to use alloys having a body-centred cubic crystal structure (referred to hereinafter as “B.C.C. alloys”), for example, alloys having the general formula TiVCr or TiVMn. Such B.C.C. alloys and their use in the storage of hydrogen have been described, in particular, by S. W. Cho, C. S. Han, C. N. Park, E. Akiba, in J. Alloys Comp., vol. 294, p. 288, (1999), or by T. Tamura, M. Hatakeyama, T. Ebinuma, A. Kamegawa, H. Takamura, M. Okada, in J. Alloys Comp., vol 505, pp. 356-357 (2003). B.C.C. alloys typically enable hydrogen to be stored with a reversible storage capacity which may reach values of the order of 2.5% by mass, or even more. Furthermore, these B.C.C. alloys are generally still efficient after several charging and discharging cycles. Moreover, the conditions of use of B.C.C. alloys are particularly advantageous inasmuch as the charging and discharging of these materials can be effected at temperatures from ambient temperature (typically 15 à 25° C.) to 100° C., without having to use hydrogen pressures higher than 1 MPa, which makes them the materials of choice for hydrogen storage.
Nevertheless, despite these various advantages, B.C.C. alloys generally have a relatively low reactivity, with reduced kinetics of hydrogen charging and discharging. Thus, it is typically observed that a solid ingot of a B.C.C. alloy having a volume of 2 cm3 absorbs a maximum of the order of 0.1% by mass of hydrogen even when it is placed under a high hydrogen pressure and at high temperatures, for example at 3 MPa and at 250° C. for 3 days. In order to achieve the above-mentioned reversible storage capacities of the order of 2.5% by mass or more, it is necessary to use the B.C.C. alloy in a form having an adequate specific surface area capable of permitting a satisfactory exchange between the alloy and the hydrogen to be stored.
To that end, a preliminary mechanical milling of the alloys having a body-centred cubic crystal structure is generally carried out before they are brought into contact with the hydrogen. Apart from the fact that it is found to be expensive, both in terms of time and in terms of energy, such mechanical milling proves to be very difficult to implement, bearing in mind the particularly high mechanical resistance which alloys having a body-centred cubic crystal structure generally have. In fact, such mechanical milling only permits the production of coarse powders, unless particularly elaborate and expensive conditions of the type of the ball milling or melt spinning techniques are used, these being incompatible with a quantitative alloy preparation which can reasonably be exploited on an industrial scale. Thus, the powders obtained in accordance with the milling techniques applicable on an industrial scale typically have, at best, a grain size of the order of from 300 μm to 500 μm.
This grain size is found to be unsatisfactory in bringing about a really efficient incorporation of hydrogen, and a subsequent activation of the material is usually required, in particular in order further to improve the specific surface area. This activation generally uses a treatment of the alloy at high temperatures. Typically, it is recommended to subject the powder to an activation pretreatment, for example, in accordance with a method of the same type as those described by Cho et al. in J. Alloys Comp. vol. 45, pp. 365-357, (2002), which consists in subjecting the powder to the following conditions:                treatment under high vacuum at 500° C. for 1 hour;        cooling to ambient temperature;        placing under a hydrogen pressure of 5 MPa for 1 hour;        repetition of the above steps at least three times; and        placing under a final vacuum at 500° C. for one hour.The necessity for such pretreatment of the powder makes the storage process even more burdensome, which is reflected, in particular, in terms of costs.        